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ELEC 6740 Electronics ELEC 6740 Electronics ManufacturingManufacturing
Chapter 9: Solder Paste and Chapter 9: Solder Paste and Its ApplicationIts Application
R. Wayne JohnsonR. Wayne JohnsonAlumni ProfessorAlumni Professor
Auburn UniversityAuburn University334334--844844--18801880
OutlineOutline1.1. IntroductionIntroduction2.2. Solder Paste PropertiesSolder Paste Properties3.3. Solder Paste Printing EquipmentSolder Paste Printing Equipment4.4. Solder Paste Printing ProcessesSolder Paste Printing Processes5.5. Paste Printing DefectsPaste Printing Defects6.6. Paste Printing VariablesPaste Printing Variables7.7. Printing for Different Types of ComponentsPrinting for Different Types of Components
IntroductionIntroduction
!! Solder paste printing is a critical step in Solder paste printing is a critical step in the SMT processthe SMT process–– Solder pasteSolder paste–– PrintersPrinters–– ProcessProcess–– DefectsDefects
Solder Paste PropertiesSolder Paste Properties
!! Solder particles in a thickened Solder particles in a thickened flux/solvent vehicleflux/solvent vehicle
Metal Composition Metal Composition --SelectionSelection
1.1. Materials of the substrate & surface mount Materials of the substrate & surface mount componentscomponents
2.2. Compatibility of the solder with the Compatibility of the solder with the metallization metallization on the substrate (Au on the substrate (Au embrittlementembrittlement, Ag leaching), Ag leaching)
3.3. Solder strength as a function of temperatureSolder strength as a function of temperature4.4. Cost (%Ag)Cost (%Ag)5.5. Lead freeLead free
Solder AlloysSolder Alloys
Highest quality for Highest quality for Au surfaces: die Au surfaces: die attachement attachement & & closuresclosures
00280E280E80Au/20Sn80Au/20Sn
Good Good wettabilitywettability, , not recommended not recommended for goldfor gold
4747134S134S--181L181L37.5Sn/37.537.5Sn/37.5Pb/25InPb/25In
99156S156S--165L165L25Pb/75In25Pb/75In
2929180S180S--209L209L50Pb/50In50Pb/50In
Less gold leaching, Less gold leaching, more ductile than more ductile than SnSn//Pb Pb alloys: Die alloys: Die attachment, attachment, closures, and closures, and general circuit general circuit assemblyassembly
1414250S250S--264L264L75Pb/25In75Pb/25In
PropertiesPropertiesDifferenceDifferenceRange (c)Range (c)AlloyAlloy
Solder AlloysSolder AlloysPropertiesPropertiesDifferenceDifferenceRange (c)Range (c)AlloyAlloy
44308S308S--312L312L5Sn/95Sn5Sn/95Sn
3434268S268S--302L302L10Sn/90Pb10Sn/90Pb
3333183S183S--216L216L50Sn/50Pb50Sn/50Pb
55183S183S--188L188L60Sn/40Pb60Sn/40Pb
Widely used tinWidely used tin--lead solders for lead solders for SMT and general SMT and general circuit assembly: circuit assembly: Low cost and Low cost and good bonding good bonding properties: Not properties: Not recommended for recommended for Ag and Au Ag and Au soldering because soldering because of high leach rate.of high leach rate.
00183E183E63Sn/37Pb63Sn/37Pb
Solder AlloysSolder AlloysPropertiesPropertiesDifferenceDifferenceRange (c)Range (c)AlloyAlloy
00309E309E1Sn/97.5Pb1Sn/97.5Pb/1.5Ag/1.5Ag
2222268S268S--290L290L10Sn/88Pb/10Sn/88Pb/2Ag2Ag
TinTin--lead solders lead solders containing small containing small amounts of Ag to amounts of Ag to minimize leaching minimize leaching of Ag conductors of Ag conductors and leads: Not and leads: Not recommended for recommended for Au: Au: SnSn//PbPb/Ag /Ag (62/36/2) is (62/36/2) is strongest tinstrongest tin--lead lead solder.solder.
00179E179E62Sn/36Pb/62Sn/36Pb/2Ag2Ag
Solder AlloysSolder AlloysPropertiesPropertiesDifferenceDifferenceRange (c)Range (c)AlloyAlloy
Low temperature Low temperature eutectic with high eutectic with high strengthstrength
00138E138E42Sn/58Bi42Sn/58Bi
1919221S221S--240L240L95Sn/5Ag95Sn/5Ag
Widely used tinWidely used tin--silver solders silver solders providing very providing very strong, lead free strong, lead free joints: Minimizes joints: Minimizes Ag leaching: Not Ag leaching: Not recommended for recommended for AuAu
00221E221E96.5Sn/3.596.5Sn/3.5AgAg
Metal ContentMetal Content
0.00200.00200.0090.0097575
0.00250.00250.0090.0098080
0.00350.00350.0090.0098585
0.00450.00450.0090.0099090
Reflowed Reflowed solder (in.)solder (in.)
Wet paste Wet paste thickness (in.)thickness (in.)
Metal Content Metal Content (wt. %)(wt. %)
Particle SizeParticle Size
Ratio = Surface Area
Volume= ΠR2
(4/3) ΠR3= 3/4R = 1.5/D
Particle SizeParticle Size
Solder PasteSolder Paste
Type 3 Paste: 25-45µµµµm particle size range
Paste TypesPaste Types
551515--551515202066
15152525--15152525303055
20203838--20203838404044
20204545--25254545505033
20207575--45457575808022
2020150150--757515015016016011
10% 10% Maximum Maximum Less Than Less Than (um)(um)
80% 80% Minimum Minimum Between (um)Between (um)
Less Than Less Than 1% Larger 1% Larger Than (um)Than (um)
None None Larger Larger Than (Than (um)um)
TypeType
Solder Powder Mesh Solder Powder Mesh DesignationDesignation
3.333.3318182525--1515--500500Type 5Type 5
1.931.9331313838--2020--400/+500400/+500Type 4Type 4
1.711.7135354545--2525--325/+500325/+500Type 3Type 3
1160607575--4545--200/+325200/+325Type 2Type 2
Surface Surface Area Area RatioRatio
Ave. Ave. Particle Particle SizeSize
Particle Particle Size (Size (µµµµµµµµm)m)
MeshMeshASTM ASTM B214B214
DesignationDesignationJJ--STDSTD--005005
Type vs. PitchType vs. Pitch
555530300.0060.0060.0100.010
4 or 54 or 55 or 6.65 or 6.640400.0080.0080.0150.015
335550500.0100.0100.0200.020
2 0r 32 0r 34.3 or 74.3 or 750500.0140.0140.0250.025
228880800.0250.0250.0500.050
Desired Desired Paste Paste TypeType
Stencil Stencil ApetureApeture//Max PSMax PS
Max. Max. Powder Powder size (size (µµµµµµµµm)m)
Stencil Stencil apertureaperture(in.)(in.)
Component Component pitch (in.)pitch (in.)
Flux Activators and Wetting Flux Activators and Wetting ActionAction
!! The activators in the flux promote The activators in the flux promote wetting of the molten solder to the wetting of the molten solder to the surface mount lands and component surface mount lands and component terminations by removing oxides and terminations by removing oxides and other surface contaminants.other surface contaminants.
!! Fluxes are generally mild acidsFluxes are generally mild acids
JJ--STDSTD--004004
!! Rosin basedRosin based!! Water solubleWater soluble!! Low residue or NoLow residue or No--cleanclean
RosinRosin
!! Rosin flux is primarily composed of Rosin flux is primarily composed of natural resin extracted from the oleoresin natural resin extracted from the oleoresin of pine trees and refined.of pine trees and refined.–– Rosin (R)Rosin (R)–– Rosin, mildly activated (RMA)Rosin, mildly activated (RMA)–– Rosin activated (RA)Rosin activated (RA)
!! Rarely used, very high activity levelRarely used, very high activity level
Water SolubleWater Soluble
!! Organic acids (OA)Organic acids (OA)!! Must be cleaned after solderingMust be cleaned after soldering!! Formulated with a glycol baseFormulated with a glycol base
NoNo--cleanclean
!! Natural resins other than rosin types Natural resins other than rosin types and/or and/or synethicsynethic resinsresins
!! Varying ‘solids’ contentVarying ‘solids’ content–– Impacts amount of flux residueImpacts amount of flux residue–– Some leave no visible residueSome leave no visible residue
!! Residue is nonResidue is non--corrosive, noncorrosive, non--conductive conductive and can be left on the boardand can be left on the board
Void formationVoid formation!! If the solvent in If the solvent in
the paste does the paste does not evaporate not evaporate before the before the solder melts, solder melts, gas bubbles gas bubbles can be can be entrapped in entrapped in the molten the molten solder creating solder creating a void.a void.
SOLDER BALL VOIDSSOLDER BALL VOIDSArea Array PackagesArea Array Packages
Rheology Rheology PropertiesProperties
!! ViscosityViscosity!! SlumpSlump!! TackinessTackiness!! Working lifeWorking life
ViscosityViscosity
!! Viscosity = Shear Stress/Shear Rate Viscosity = Shear Stress/Shear Rate (Pascal(Pascal--seconds)seconds)
!! The internal resistance exerted by a fluid The internal resistance exerted by a fluid to the relative motion of its partsto the relative motion of its parts
ViscosityViscosity
!! Shear rate (secShear rate (sec--11) is the rate of travel of ) is the rate of travel of the two parallel plates separated by fluid the two parallel plates separated by fluid divided by the distance between the divided by the distance between the plates (cm/s/cm)plates (cm/s/cm)
Force(Shear stressPascals)
Fluid
Viscosity as a Function of Viscosity as a Function of ProcessProcess
102
103
104
105
Vis
cosi
ty (P
oise
)
Time
Stirring
Squeegee
Thr
ough
the
scre
en
Leveli
ng103
102
10
1
0.1
0.01
Shea
r R
ate
(sec
-1)
Newtonian
Pseudoplastic Thixotropic
Dilatant
Response of Fluids to ShearResponse of Fluids to Shear
Shear Stress
ShearRate
ViscometerViscometer
Spiral (Spiral (MalcomMalcom) Viscometer) Viscometer
Schematic of Spiral InductorSchematic of Spiral Inductor
Shear RateShear Rate
Solder BallsSolder Balls1.1. Solder balls are formed by very fine powder Solder balls are formed by very fine powder
particles in the solder paste.particles in the solder paste.!! They are carried away from the main solder They are carried away from the main solder
deposit as the flux melts and flows before the deposit as the flux melts and flows before the solder itself melts.solder itself melts.
!! This happens especially when the paste is This happens especially when the paste is deposited outside the land area either by design or deposited outside the land area either by design or misregistrationmisregistration
!! These smaller particles lose contact with the larger These smaller particles lose contact with the larger solder paste deposit and when the solder melts, solder paste deposit and when the solder melts, each particle becomes a small solder ball at the each particle becomes a small solder ball at the periphery of the original paste depositperiphery of the original paste deposit
!! A A collectionm opf collectionm opf small solder balls around the small solder balls around the main solder deposit is called a ‘halo’.main solder deposit is called a ‘halo’.
Solder BallsSolder Balls2.2. Solder balls are also formed when the oxide Solder balls are also formed when the oxide
layer on the layer on the nsurface nsurface of the solder powder of the solder powder particles is so thick the flux and any activator particles is so thick the flux and any activator in the paste are not sufficient to remove it.in the paste are not sufficient to remove it.
!! Since the oxide cannot melt at soldering Since the oxide cannot melt at soldering temperatures, they are pushed aside as a solder temperatures, they are pushed aside as a solder ball by the surrounding oxideball by the surrounding oxide--free molten solder.free molten solder.
!! Solder balls formed in this manner are larger than Solder balls formed in this manner are larger than those formed by the 1those formed by the 1stst mechanism because of the mechanism because of the presence of surface oxide which is less dense than presence of surface oxide which is less dense than the metal.the metal.
Solder BallsSolder Balls
!! Improper handlingImproper handling!! Excessive baking/preheat prior to Excessive baking/preheat prior to reflowreflow!! Particle rubbing (Fretting corrosion)Particle rubbing (Fretting corrosion)!! NoNo--clean more likely to have solder ballsclean more likely to have solder balls
–– Less aggressive fluxLess aggressive flux
TestingTesting
!! Print paste onto nonPrint paste onto non--metallic substrate metallic substrate (ceramic, glass, FR(ceramic, glass, FR--4)4)
!! ReflowReflow!! Inspect for solder ballsInspect for solder balls
Solder BallsSolder Balls
PrintabilityPrintability
1.1. Weight 5 clean Weight 5 clean dummay dummay boards (W1) boards (W1) and after (W2) the paste is printed.and after (W2) the paste is printed.
2.2. Determine the weight of paste deposited Determine the weight of paste deposited (W2(W2--W1)W1)
3.3. Measure and record the height at 4 Measure and record the height at 4 predetermined points on each substratepredetermined points on each substrate
4.4. Perform steps 1Perform steps 1--3 for freshly removed 3 for freshly removed solder paste and solder paste exposed to solder paste and solder paste exposed to the atmosphere for 4 hours.the atmosphere for 4 hours.
PrintabilityPrintability1.1. The solder paste weight should not vary by The solder paste weight should not vary by
more than 10% among the average more than 10% among the average measurements taken on one substratemeasurements taken on one substrate
2.2. The paste height should not vary by more The paste height should not vary by more than +than +-- 1mil among the average 1mil among the average measurements taken on one substratemeasurements taken on one substrate
3.3. The solder paste pattern should have uniform The solder paste pattern should have uniform coverage, without stringing and without coverage, without stringing and without separation of flux and solder, and should separation of flux and solder, and should print without forming a peak.print without forming a peak.
Printing EquipmentPrinting Equipment
Printing EquipmentPrinting Equipment
Printing EquipmentPrinting Equipment
Printer SelectionPrinter Selection
!! Manual vs. computer control of Manual vs. computer control of parameterparameter
!! Stencil sizeStencil size!! PWB sizePWB size!! Print modePrint mode!! AlignmentAlignment
Print VariablesPrint Variables
Printing ParametersPrinting Parameters
Screens & StencilsScreens & Stencils
Stencils & ScreensStencils & Screens
Stencils & ScreensStencils & Screens
ScreensScreens
ScreensScreens
StencilsStencils
!! Stainless steelStainless steel!! NickelNickel!! BrassBrass
StencilsStencils
StencilsStencils
Flexible Metal Mask StencilFlexible Metal Mask Stencil
Frame for Stretching Frame for Stretching StencilsStencils
Stencil StorageStencil Storage
Stencil FormingStencil Forming
!! Chemical EtchChemical Etch–– ElectropolishedElectropolished–– Ni PlatedNi Plated
!! Laser CutLaser Cut–– ElectropolishedElectropolished–– Ni PlatedNi Plated
!! Ni ElectroformedNi Electroformed
Chem Chem EtchEtch
As-etched Electropolished
Laser CutLaser Cut
Laser Cut ApertureLaser Cut Aperture
Ni ElectroformedNi Electroformed
Comparison of Apertures to Comparison of Apertures to Solder ParticlesSolder Particles
Print ResolutionPrint Resolution
Stencil TypeStencil Type
PrintingPrinting1.1. No matter which method of application is used, be No matter which method of application is used, be
sure that the solder paste has been stored properly. sure that the solder paste has been stored properly. A tightly sealed, unopened container of solder paste A tightly sealed, unopened container of solder paste generally can be stored for 6 months at 4generally can be stored for 6 months at 4--2929ooC. C. Shelf life is flux dependent. It is better to use the Shelf life is flux dependent. It is better to use the freshest paste possible. If opened, store in a freshest paste possible. If opened, store in a refrigerated environment.refrigerated environment.
2.2. Use only fresh paste every day. To accomplish this, Use only fresh paste every day. To accomplish this, use small jars that contain only 1 day’s worth of use small jars that contain only 1 day’s worth of paste or transfer paste from large jars as needed for paste or transfer paste from large jars as needed for the day and put the rest back in the refrigerator. the day and put the rest back in the refrigerator. This helps improve pasteThis helps improve paste--related yield.related yield.
PrintingPrinting3.3. Allow refrigerated container to reach room Allow refrigerated container to reach room
temperature before use. It may temperature before use. It may ber ber advisable advisable to take the paste out of the refrigerator the to take the paste out of the refrigerator the night before for the next day’s use to avoid night before for the next day’s use to avoid the wait.the wait.
4.4. Check the solder paste for solder ball Check the solder paste for solder ball characteristics and viscosity.characteristics and viscosity.
5.5. When all solder paste printing is complete, When all solder paste printing is complete, wash the screen or stencil with the wash the screen or stencil with the appropriate solvents. appropriate solvents.
6.6. Discard (hazardous material) any used paste.Discard (hazardous material) any used paste.
Screen PrintingScreen Printing
!! Typically offTypically off--contactcontact!! Lower viscosity for flow through wiresLower viscosity for flow through wires!! Screen and PWB should be parallel Screen and PWB should be parallel
within 0.002”within 0.002”!! SnapSnap--off distance set to 0.030” (typical)off distance set to 0.030” (typical)
Screen PrintingScreen Printing
Stencil PrintStencil Print
PWB
Stencil Solder
MetalBlade
Squeegee
Stencil PrintingStencil Printing
Stencil PrintingStencil Printing
SeparationSeparation
Step StencilStep Stencil
Step StencilsStep Stencils
Stencils vs. ScreensStencils vs. Screens
DispensingDispensing
!! Low volume, slowLow volume, slow!! RepairRepair!! Special requirementsSpecial requirements!! Clogged needlesClogged needles
–– Lower viscosityLower viscosity
Printing DefectsPrinting Defects
Visual InspectionVisual Inspection
Visual InspectionVisual Inspection
Vision InspectionVision Inspection
Solder Paste ViscositySolder Paste Viscosity!! Dispensing: 200,000 Dispensing: 200,000 –– 450,000 (450,000 (centipoisecentipoise))!! Screening: 450,000 Screening: 450,000 -- 800,000 800,000 cPcP!! Stenciling: 750,000 Stenciling: 750,000 –– 950,000 950,000 cP cP (50mil pitch)(50mil pitch)!! Stenciling: 900,000 Stenciling: 900,000 –– 1,200,000 1,200,000 cP cP (fine pitch)(fine pitch)!! Temperature effectsTemperature effects!! Paste shearingPaste shearing!! Moisture (water soluble paste)Moisture (water soluble paste)
Print ThicknessPrint Thickness
!! Stencil thicknessStencil thickness!! PressurePressure
–– ScoopingScooping!! Blade typeBlade type
–– MetalMetal!! StencilsStencils
–– RubberRubber!! Screens & StencilsScreens & Stencils
SqueegeesSqueegees
Print OrientationPrint Orientation
Print SpeedPrint Speed
!! Function of pasteFunction of paste!! Fast print speed can cause Fast print speed can cause planing planing of the of the
squeegee, resulting in skipssqueegee, resulting in skips!! Slow speed generally provides better Slow speed generally provides better
prints, but can lead to ragged edges or prints, but can lead to ragged edges or smearing of too slow.smearing of too slow.
!! Production favors fast speedsProduction favors fast speeds
Stencil ApertureStencil Aperture
Print ThicknessPrint Thickness
IPC Stencil Design RulesIPC Stencil Design Rules
Part Type Aspect Ratio Range
Area Ratio Range
PLCC (50 mil pitch) 2.3 to 3.8 0.88 to 1.48QFP (25 mil pitch) 1.7 to 2.0 0.71 to 0.83QFP (20 mil pitch) 1.7 to 2.0 0.69 to 0.83QFP (16 mil pitch) 1.6 to 2.0 0.68 to 0.86QFP (12 mil pitch) 1.5 to 2.0 0.65 to 0.86
0402 N/A 0.84 to 1.000201 N/A 0.66 to 0.89
BGA (50 mil pitch) N/A 0.93 to 1.25BGA (40 mil pitch) N/A 0.67 to 0.78BGA (20 mil pitch) N/A 0.69 to 0.92
DefinitionsDefinitions
Area of Aperture OpeningArea of Aperture Wall
Area RatioArea Ratio
Width of ApertureStencil Thickness
Aspect RatioAspect Ratio
Rule of thump:
Area Ratio > 0.66
Paste in HolePaste in Hole
)4/()(2Re 22 TxxvolumepastequiredRVP DD lh −∏==
Dh = plated through hole diameter
Dl = lead diameter
T = the board thickness
Hole should be 0.012” bigger than lead diameteror 0.010” bigger than the diagonal dimension of the lead.
Paste in HolePaste in Hole
Paste in HolePaste in Hole
!! Aperture larger than PWB padAperture larger than PWB pad–– Printing on solder maskPrinting on solder mask
!! Potential for solder ballsPotential for solder balls
